1. Field of the Invention
The present invention relates to a method of pressing a disk that can be used as the disk for recording medium such as magnetic disk, a glass suitable for said method, a method of pressing a hard disk substrate from said glass by using a press-mold, and a method of pressing a hard disk substrate from said glass by using a press-mold with a release layer.
2. Description of the Related Art
In the field of magnetic disk, technical development is being made rapidly toward increasing the recording density and transfer rate. Nowadays, it is urgent to develop a high-speed disk for rapid transfer. Hence, there is a demand for a disk material with a high specific rigidity which will not vibrate during high-speed running. Conventional aluminum disks (referred to as aluminum substrate hereinafter) have a specific rigidity of 26.7 (Young""s modulus [72 GPa] divided by density [2.7 g/cm3]). It is said that aluminum disks need more than twice that specific rigidity if they are to be used at high speeds of 10000 rpm. The only way to double the specific rigidity of aluminum disks is to combine aluminum with ceramics (MMC). This is not practical from the standpoint of production cost.
On the other hand, glass disks used in 2.5-inch size (referred to as glass substrate hereinafter) are attracting attention because it is easy to increase their specific rigidity. This object is achieved by heating glass at an adequate temperature, thereby causing a crystalline phase with a high Young""s modulus to separate out. The resulting glass ceramic has a high Young""s modulus. For example, Japanese Patent Laid-open Nos. 329440/1994, 111024/1996, and 221747/1996 disclose a process for causing lithium dioxide crystals and xcex1-quartz crystals to separate out. And Japanese Patent Laid-open No. 77531/1998 discloses a process for causing spinel crystals to separate out, thereby increasing Young""s modulus to 109-144 GPa and specific rigidity to 36-47.
The disadvantage of the disclosed technology is that crystallization increases the specific rigidity of glass but it also gives rise to a composite structure (composed of the hard crystalline phase and the soft glass phase). Such a composite structure produces minute steps at the time of polishing, making it difficult to obtain a super-mirror required of disks.
There is a way to increase the specific rigidity of glass per se by incorporation with a rare earth element which improves Young""s modulus of glass. The disadvantage of this technology is that incorporation with a rare earth element increases not only Young""s modulus but also specific gravity, with the result that the specific rigidity of glass does not increase as expected.
Thus, as a means to increase Young""s modulus without remarkably increasing the specific gravity of glass, using an oxynitride glass, with nitrogen replaced by oxygen in glass, is being investigated. However, an oxynitride glass is high in viscosity at high temperatures, and its conversion into a disk by industrial pressing was difficult.
The present inventors found an optimum pressing condition, a press-mold, and a release layer required to make the oxynitride glass into disks. Thus, it is an object of the present invention to provide an oxynitride glass disk useful as a high-performance magnetic disk having a high specific rigidity.
The gist of the present invention resides in a method of producing a disk having a radius of r and a thickness of 2h from an oxynitride glass by pressing, said method being characterized in that the pressing load (F), the pressing temperature (T), and the pressing time (t) are defined by the expression below.                     1900        ≥        T        ≥                              100            xc3x97                                          log                10                            ⁡                              (                                                      0.045                    xc3x97                    π                    xc3x97                                          r                      4                                                                            t                    xc3x97                                          h                      2                                        xc3x97                    F                                                  )                                              +          A                                    (        1        )            
where,
F: pressing load (N)
T: pressing temperature (xc2x0 C.)
t: pressing time (s)
xcfx80: ratio of the circumference to its diameter
r: radius of disk formed by pressing (mm)
h: half a thickness of disk formed by pressing (mm)
A: constant depending on glass composition (xc2x0 C.) defined by 1450xc2x0 C.xe2x89xa6Axe2x89xa6650xc2x0 C.
The present invention also covers an oxynitride glass disk produced by pressing according to the above-mentioned method. A preferred material for the disk is an oxynitride glass represented by M1xe2x80x94Alxe2x80x94Sixe2x80x94Oxe2x80x94N (M1 denoting one or more species selected from Ca, Mg, Y, Gd, Ce, and La), an oxynitride glass represented by M2xe2x80x94Sixe2x80x94Oxe2x80x94N (M2 denoting Ca and/or Mg), or an oxynitride glass composed of more than one kind thereof.
It is particularly desirable that M1 is one species selected from Ca, Mg, Y, Gd, and Ce and that the oxynitride glass of this composition contains N in an amount defined by 5 eq %xe2x89xa6Nxe2x89xa625 eq %, with O+N=100 eq %, the amount of metallic components (M1, Al, and Si) is within the hatched area in the diagrams shown in FIGS. 1 to 5.
The present invention is also directed to a press-mold used for said oxynitride glass, said press-mold being made of graphite. Graphite has good heat resistance and prevents glass from sticking thereto. Graphite having a bulk density higher than 1.7 g/cm3 is desirable for the press-mold so that it helps the press-molded product to have a smooth surface.
The present invention is also directed to a press-mold used for said oxynitride glass, said press-mold being made of glassy carbon. Like graphite, glassy carbon has good heat resistance and prevents glass from sticking thereto. Glassy carbon having a bulk density higher than 1.5 g/cm3 is desirable for the press-mold so that it helps the press-molded product to have a smooth surface.
The press-mold should preferably have surface smoothness such that the maximum height of irregularities (Rmax) is 0.1-5 xcexcm and the center line average roughness (Ra) is 0.01-0.5 xcexcm (measured according to JIS B0601). This press-mold helps the press-molded product to have a smooth surface.
According to a preferred embodiment of the present invention, a discoid product with accurate dimensions, A mm in diameter and B mm in thickness, may be obtained by using a circular press-mold with a concave surface whose radius of curvature (R1) along the center line of the roughness curve (measured according to JIS B0601) is greater than 1200xc3x97A mm.
The present invention covers a method of pressing an oxynitride glass by using the above-mentioned press-mold.
The press-molded product of oxynitride glass obtained by using the above-mentioned press-mold has surface roughness at its center such that the maximum height of irregularities (Rmax) is smaller than 1 xcexcm and the center line average roughness (Ra) is smaller than 0.1 xcexcm (measured according to JIS B0601). This surface state only needs finish polishing for the molded product to be completed.
The present invention is directed to a discoid molded product (A mm in diameter and B mm in thickness) obtained by using the circular press-mold (defined in claim 6). This molded product has a curved concave or convex surface whose radius of curvature (R2) along the center line of roughness curve (measured according to JIS B0601) is greater than 1200xc3x97A mm, and the molded product also has a thickness of 1.0-1.1xc3x97B mm. With such accurate dimensions, the molded product dispenses with polishing to remove much glass and helps reduce polishing cost.
The present invention is directed to a method of pressing an oxynitride glass by using a press-mold. This method employs a release layer containing carbon and/or boron nitride which is interposed between the press-mold and the oxynitride glass being press-molded. The release layer helps the oxynitride glass to be demolded easily after molding, and it also keeps the content of nitrogen in the molded oxynitride glass higher than 90% (by mass) of the original content before pressing.
According to a preferred embodiment, this release layer may be formed by applying a release agent to the surface of the press-mold and/or the surface of the oxynitride glass before pressing, said release agent containing carbon powder and/or boron nitride powder having an average particle diameter of 0.01-100 xcexcm. The release layer may also be formed by deposition of carbon and/or boron nitride on the surface of the press-mold and/or the surface of the oxynitride glass before pressing.
The release layer should preferably be formed from a release agent in an amount of 1xc3x9710xe2x88x925-5xc3x9710xe2x88x921 g per unit area (cm2) of the surface of the molded product. Also, the release layer formed by deposition should preferably have a thickness of 0.001-10 xcexcm.
The present invention is directed to a release layer used for pressing an oxynitride glass. This release layer contributes to a press-molded product of oxynitride glass having good surface smoothness. It is a film of amorphous carbon and/or amorphous carbon nitride formed by deposition. The deposited film of amorphous carbon and/or amorphous carbon nitride exhibits outstanding releasing performance at high temperatures in the pressing of an oxynitride glass. The deposited film firmly adheres to the molding surface of the press-mold and has a uniform thickness. Therefore, it prevents the oxynitride glass from sticking to the press-mold and hence helps give a press-molded product of oxynitride glass having good surface smoothness. The release layer should preferably have a thickness of 1-1000 nm.
The present invention is directed to a method of forming the release layer for pressing an oxynitride glass. This release layer is formed by physical deposition on the molding surface of the press-mold. The deposited film is compact and firmly adheres to the smooth molding surface of the press-mold and has a uniform thickness. Therefore, it exhibits good releasing performance and helps give a press-molded product of oxynitride glass having good surface smoothness.
The physical deposition should preferably be carried out by sputtering with amorphous carbon and/or amorphous carbon nitride under the following conditions.
Temperature of the molding surface of the press-mold: 0-400xc2x0 C.
Gas pressure for sputtering 0.1-3 Pa
Power density for sputtering: 1-10 W/cm2